This invention relates to the manufacture of a powdered composition suitable for use in the manufacture of cubic boron nitride abrasive compacts, and specifically to compacts with enhanced wear resistance, and increased chip resistance.
Boron nitride exists typically in three crystalline forms, namely cubic boron nitride (CBN), hexagonal boron nitride (hBN) and wurtzitic cubic boron nitride (wBN). Cubic boron nitride is a hard zinc blende form of boron nitride that has a similar structure to that of diamond. In the CBN structure, the bonds that form between the atoms are strong, mainly covalent tetrahedral bonds. Methods for preparing CBN are well known in the art. One such method is subjecting hBN to very high pressures and temperatures, in the presence of a specific catalytic additive material, which may include the alkali metals, alkaline earth metals, lead, tin and nitrides of these metals. When the temperature and pressure are decreased, CBN may be recovered.
CBN has wide commercial application in machining tools and the like. It may be used as an abrasive particle in grinding wheels, cutting tools and the like or bonded to a tool body to form a tool insert using conventional electroplating techniques.
CBN may also be used in bonded form as a CBN compact. CBN compacts tend to have good abrasive wear, are thermally stable, have a high thermal conductivity, good impact resistance and have a low coefficient of friction when in contact with iron containing metals.
Diamond is the only known material that is harder than CBN. However, as diamond tends to react with certain materials such as iron, it cannot be used when working with iron containing metals and therefore use of CBN in these instances is preferable.
CBN compacts comprise sintered masses of CBN particles. When the CBN content exceeds 80 percent by volume of the compact, there is a considerable amount of CBN-to-CBN contact and bonding. When the CBN content is lower, e.g. in the region of 40 to 60 percent by volume of the compact, then the extent of direct CBN-to-CBN contact and bonding is less.
CBN compacts will generally also contain a binder phase for example aluminium, silicon, cobalt, nickel, and titanium.
When the CBN content of the compact is less than 70 percent by volume there is generally present another hard phase, a secondary phase, which may be ceramic in nature. Examples of suitable ceramic hard phases are carbides, nitrides, borides and carbonitrides of a Group 4, 5 or 6 (according to the new IUPAC format) transition metal, aluminium oxide, and carbides such as tungsten carbide and mixtures thereof. The matrix constitutes all the ingredients in the composition excluding CBN.
In U.S. Pat. No. 6,316,094, a variety of mixing techniques are employed to effect the mixing of the matrix materials and the CBN prior to sintering, for example ultrasonic mixing, ball milling and mixing using an attritor. Mixing using an attritor was claimed to result in a decreased tool life when compared to most other mixing methods.
CBN compacts may be bonded directly to a tool body in the formation of a tool insert or tool. However, for many applications it is preferable that the compact is bonded to a substrate/support material, forming a supported compact structure, and then the supported compact structure is bonded to a tool body. The substrate/support material is typically a cemented metal carbide that is bonded together with a binder such as cobalt, nickel, iron or a mixture or alloy thereof. The metal carbide particles may comprise tungsten, titanium or tantalum carbide particles or a mixture thereof.
A known method for manufacturing the CBN compacts and supported compact structures involves subjecting an unsintered mass of CBN particles, to high temperature and high pressure conditions, i.e. conditions at which the CBN is crystallographically stable, for a suitable time period. A binder phase may be used to enhance the bonding of the particles. Typical conditions of high temperature and pressure (HTHP) which are used are temperatures in the region of 1100° C. or higher and pressures of the order of 2 GPa or higher. The time period for maintaining these conditions is typically about 3 to 120 minutes.
The sintered CBN compact, with or without substrate, is often cut into the desired size and/or shape of the particular cutting or drilling tool to be used and then mounted on to a tool body utilising brazing techniques.
CBN compacts are employed widely in the manufacture of cutting tools for finish machining of hardened steels, such as case hardened steels, ball-bearing steels and through hardened engineering steels. In addition to the conditions of use, such as cutting speed, feed and depth of cut, the performance of the CBN tool is generally known to be dependent on the geometry of the workpiece and in particular, whether the tool is constantly engaged in the workpiece for prolonged periods of time, known in the field as “continuous cutting”, or whether the tool engages the workpiece in an intermittent manner, generally known in the field as “interrupted cutting”.
Depending on the workpiece geometry, it is common for the CBN tool to experience both continuous and interrupted cutting within a process cycle and furthermore, the ratio of continuous to interrupted cutting varies widely in the field. After extensive research in this field it was discovered that these different modes of cutting place very different demands on the CBN material comprising the cutting edge of the tool. The main problem is that the tools tend to fail catastrophically by fracturing or chipping, exacerbated by an increasing demand in the market for higher productivity through increased cutting speeds and hence have a reduced tool life.